Although the National Institute of Occupational Safety and Health (NIOSH) has identified a clear epidemiological link between repetitive arm motion and shoulder disorders in the workplace, there are few scientific data available regarding the biomechanics of this connection. Shoulder overuse injuries may initially manifest as diffuse shoulder pain, however, the vast majority reflect pathology of the rotator cuff tendons in the subacromial space. The long-term goal of our research agenda is to identify the important biomechanical mechanisms associated with work related rotator cuff injuries and subsequently develop intervention and prevention strategies that address these mechanisms. The objective of this application is to identify the link between the integrity of the rotator cuff mechanism and its loading environment. To accomplish this objective, we have developed three specific aims. Our first specific aim is to establish the in-vivo kinematic response to diminished rotator cuff function in a nerve block model. Our second specific aim is to establish the in-vivo kinematic response to high risk occupational work. Our third specific aim is to establish the biomechanical consequences of altered in-vivo kinematics. It is our expectation that the proposed research plan will identify the direct consequences of diminished cuff function observed in the workplace. These results will be significant because they represent the next step towards understanding the progression from fatigue and weakness to abnormal loading. Since there is strong evidence to support the concept that abnormal loading may lead to a progression from acute tendinitis to chronic rotator cuff tears, the results from this proposal will also serve as the basis for more effective treatment and prevention strategies.